Refrigeration systems are prevalent in our everyday life. In the beverage industry, refrigeration systems are found in vending machines, glass door merchandisers ("GDMs") and dispensers. In the past, these units have kept beverages or containers containing a beverage cold using conventional vapor compression (Rankine cycle) refrigeration apparatus. In this cycle, the refrigerant in the vapor phase is compressed in a compressor, causing an increase in temperature. The hot, high pressure refrigerant is then circulated through a heat exchanger, called a condenser, where it is cooled by heat transfer to the surrounding environment. As a result of the heat transfer to the environment, the refrigerant condenses from a gas to a liquid. After leaving the condenser, the refrigerant passes through a throttling device where the pressure and temperature both are reduced. The cold refrigerant leaves the throttling device and enters a second heat exchanger, called an evaporator, located in the refrigerated space. Heat transfer in the evaporator causes the refrigerant to evaporate or change from a saturated mixture of liquid and vapor into a superheated vapor. The vapor leaving the evaporator is then drawn back into the compressor, and the cycle is repeated. A variation of the vapor compression cycle as outlined above is the transcritical carbon dioxide vapor compression cycle where the condenser is replaced with an ultra-high pressure gas cooler and phase change does not occur.
Stirling coolers have been known for decades. Briefly, a Stirling cycle cooler compresses and expands a gas (typically helium) to produce cooling. This gas shuttles back and forth through a regenerator bed to develop much larger temperature differentials than the simple compression and expansion process affords. A Stirling cooler uses a displacer to force the gas back and forth through the regenerator bed and a piston to compress and expand the gas. The regenerator bed is a porous element with a large thermal inertia. During operation, the regenerator bed develops a temperature gradient. One end of the device becomes hot and the other end becomes cold. David Bergeron, Heat Pump Technology Recommendation for a Terrestrial Battery-Free Solar Refrigerator, September 1998. Patents relating to Stirling coolers include U.S. Pat. Nos. 5,678,409; 5,647,217; 5,638,684; 5,596,875; and 4,922,722.
Stirling coolers are desirable because they are nonpolluting, are efficient and have very few moving parts. The use of Stirling coolers has been proposed for conventional refrigerators. See U.S. Pat. No. 5,438,848. However, it has been recognized that the integration of free-piston Stirling coolers into conventional refrigerated cabinets requires different techniques than conventional compressor systems. D. M. Berchowitz et al., Test Results for Stirling Cycle Cooler Domestic Refrigerators, Second International Conference. To date, the use of Stirling coolers in beverage vending machines, GDMs and dispensers is not known.
Therefore, a need exists for adapting Stirling cooler technology to conventional beverage vending machines, GDMs, dispensers and the like.